1. Field of the Invention
The present invention relates to a dielectric filter and a dielectric duplexer using TM multi-mode dielectric resonators.
2. Description of the Related Art
Hitherto, TM double-mode dielectric resonators with cross-shaped dielectric members disposed in a cavity having a conductor on its outer surface have been used in a bandpass-type dielectric filter. An example of this type of dielectric filter is shown in FIG. 6.
Referring to FIG. 6, a dielectric filter generally designated by 101 has four TM double-mode dielectric resonators 102, 103, 104 and 105 aligned in a row with their openings facing in the same direction, and metallic panels 106 and 107 to cover the openings of the resonators 102 through 105.
The dielectric resonator 102 is constructed in the following manner. A cavity 102a having openings at its forward and back ends and a cross-shaped dielectric member 102XY are integrally formed of the same dielectric material. Further, a conductor 102b is disposed on the outer surfaces, except for the openings, of the cavity 102a. The dielectric member 102XY is formed of a horizontal dielectric portion 102X and a vertical dielectric portion 102Y, so that the single TM double-mode dielectric resonator 102 can form a two-stage resonator. The TM double-mode dielectric resonators 103, 104 and 105 are constructed in a manner similar to the above-described dielectric resonator 102.
An input loop 108 and an output loop 109 are attached to the panel 106 and connected to an external circuit via a coaxial connector (not shown). Fixed to the panel 107 are coupling loops 107a, 107b, 107c and 107d each for coupling adjacent dielectric resonators.
The operation of the dielectric filter 101 constructed as described above will now be explained. High-frequency power is first supplied to the dielectric filter 101 to magnetically couple the input loop 108 and the vertical dielectric portion 102Y serving as a first-stage resonator. Due to this magnetic coupling, the TM110Y mode shown in FIG. 7B is excited in the vertical dielectric portion 102Y, and is then electromagnetically coupled within the dielectric member 102XY to the horizontal dielectric portion 102X used as a second-stage resonator. Accordingly, a TM110X mode illustrated in FIG. 7A is further excited in the horizontal dielectric portion 102X, and is then magnetically coupled to the coupling loop 107a.
The coupling loop 107a opposedly faces not only the horizontal dielectric portion 102X used as the second-stage resonator, but also the horizontal dielectric portion 103X serving as a third-stage resonator. Thus, the coupling loop 107a magnetically coupled to the horizontal dielectric portion 102X is further magnetically coupled to the horizontal dielectric portion 103X. Because of this coupling, the TM110X mode shown in FIG. 7A is excited in the horizontal dielectric portion 103X, and is further electromagnetically coupled within the dielectric member 103XY to the vertical dielectric portion 103Y used as a fourth-stage resonator. Due to this coupling, the TM110Y mode illustrated in FIG. 7B is excited in the vertical dielectric portion 103Y, and is then magnetically coupled to the coupling loop 107b.
The coupling loop 107b is connected to the coupling loop 107c, and as in the case with the loop 107b opposedly facing the vertical dielectric portion 103Y as the fourth-stage resonator, the loop 107c opposedly faces the vertical dielectric portion 104Y used as a fifth-stage resonator. Accordingly, the loop 107b magnetically coupled to the vertical dielectric portion 103Y is further magnetically coupled to the vertical dielectric portion 104Y via the coupling loop 107c.
Thereafter, in a manner similar to the above procedure, the vertical dielectric portion 104Y is electromagnetically coupled to the horizontal dielectric portion 104X serving as a sixth-stage resonator. The horizontal dielectric portion 104X is further magnetically coupled via the coupling loop 107d to the horizontal dielectric portion 105X used as a seventh-stage resonator, which is then electromagnetically coupled to a vertical dielectric portion 105Y as an eighth-stage resonator. Finally, the vertical dielectric portion 105Y is magnetically coupled to the output loop 109, and the generated magnetic field is then output via a coaxial connector (unillustrated).
However, in the above conventional type of multi-stage dielectric filter shown in FIG. 6, the shape of the loops for coupling the TM110X mode generated in the horizontal dielectric portions, such as the coupling loops 107a and 107d, greatly differ from that of the loops for coupling the TM110Y mode generated in the vertical dielectric portions, such as the coupling loops 107b and 107c. This requires two types of molds for producing the above types of loops, thereby increasing the cost.
The above-mentioned increased cost particularly originates from the loops for coupling the vertical dielectric portions which are formed in a curved, complicated shape, in contrast to the simply shaped loops for coupling the horizontal dielectric portions. Further, the loops for coupling the vertical dielectric portions are easily deformed due to their complicated shape, which may disadvantageously change the coupling amount between the vertical dielectric portions, thereby further varying the filter characteristics.
Moreover, although the coupling loops 107a through 107d are mounted on the same panel 107, the input and output loops 108 and 109 are unable to be attached onto the panel 107 due to lack of space, since the loops 108 and 109 would otherwise overlap with the coupling loops 107a and 107d, respectively.
Additionally, the dielectric filter 101 has frequency adjustment means for each of the vertical dielectric portion and the horizontal dielectric portion of each TM double-mode dielectric resonator, and coupling control means for controlling the coupling coefficient between the vertical and horizontal dielectric portions (neither of these means are shown in FIG. 6). Generally, threaded metallic or dielectric rods are used as the frequency adjustment means and coupling control means.
The frequency adjustment means and the coupling-coefficient control means are mounted on the panel 106. More specifically, the above means for the TM double-mode dielectric resonator 102 are attached in the vicinity of the input loop 108, while the means for the resonator 105 are fixed in the vicinity of the output loop 109.
Consequently, when the frequency adjustment means and the coupling-coefficient control means provided near the input loop 108 are actuated, the coupling coefficient between the input loop 108 and the vertical dielectric portion 102Y may disadvantageously be changed. Likewise, when the frequency adjustment means and control means in the vicinity of the output loop 109 are actuated, the coupling coefficient between the output loop 109 and the vertical dielectric portion 105Y may unfavorably be changed.